Reference signal allocation for flexible data lengths

ABSTRACT

Physical layer characteristics, e.g., reference signal density, reference signal distribution, data parameters, etc., are defined in the physical layer for sub-frames of a transmission time interval (TTI) allocated to data packet(s) based on the number of allocated sub-frames. The flexibility provided by the solution presented herein enables the associated wireless system to better define those physical layer characteristics necessary to meet signal quality and system requirements without unnecessarily overburdening the system overhead. Thus, the reference signal overhead may be reduced, which leads to reduced system overhead and/or increased spectrum efficiency.

This application claims priority to Provisional U.S. Patent ApplicationSer. No. 61/877,466 filed 13 Sep. 2013, the disclosure of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The invention disclosed herein relates generally to reference signalallocation in data packets transmitted in a wireless communicationsystem, and more particularly to the flexible allocation of physicallayer characteristic(s) within variable length data packets.

BACKGROUND

Over the past several decades, radio communications have significantlyimpacted the ways in which people go about their daily lives. Sensorsystems use radio communications to exchange data and commands. Peopleuse radio communications to chat with other people (e.g., via voicecalls, texting, instant messaging, video calls, etc.), stream videos,listen to music, download information, send/receive photos, etc. Such awide variety of different types of radio communications typically have awide range of requirements. Efficient, flexible, and low cost designscapable of meeting all of these requirements are desirable, and will beimportant for meeting future radio communication demands.

One way wireless devices strive to meet such requirements is through theuse of pilot or reference signals, which are transmitted by atransmitter and known by a receiver. Radio signals experience variousdistortions when propagated from the transmitter to the receiver viaradio propagation channels. The transmission of the pilot or referencesignals, e.g., with the data signals, enables the receiver to moreaccurately recover the transmitted signal. More particularly, thereceiver uses the known pilot/reference signal(s) to estimate the radiochannel between the transmitter and the receiver, and uses the estimatedradio channel to perform equalization on the data signal to accuratelyrecover the transmitted data signal. Because the receiver relies heavilyon pilot/reference signals to accurately process received data, andbecause the use of such pilot/reference signals requires a fair amountof overhead, there is much interest in optimizing the use of suchsignals.

SUMMARY

The solution presented herein defines how physical layercharacteristics, e.g., reference signal density, reference signaldistribution, data parameters, etc., defined in the physical layer forsub-frames of a transmission time interval (TTI) allocated to datapacket(s) may vary based on the number of allocated sub-frames. Theflexibility provided by the solution presented herein enables theassociated wireless system to better define those physical layercharacteristics necessary to meet signal quality and system requirementswithout unnecessarily overburdening the system overhead. Thus, thesolution presented herein enables a reduction in the reference signaloverhead, which leads to reduced system overhead and/or increasedspectrum efficiency.

One exemplary embodiment provides a method of transmitting data from awireless communication device to a remote device using one or moreallocated sub-frames of a transmission time interval (TTI). The methodcomprises allocating one or more sub-frames to a data packet anddefining one or more physical layer characteristics of the allocatedsub-frames based on the number of allocated sub-frames. The one or morephysical layer characteristics comprise at least one of a referencesignal density, a sub-frame structure, and a data rate parameter. Thereference signal density identifies a number of reference signalsallocated to the data packet in response to the number of allocatedsub-frames. The reference signal density varies disproportionatelyrelative to the number of allocated sub-frames as the number ofallocated sub-frames varies. The sub-frame structure identifies adistribution of one or more reference signals within the data packet inresponse to the number of allocated sub-frames. The data rate parameteridentifies at least one of a coding rate and a rate matching of the datapacket in response to the number of allocated sub-frames. The data rateparameter varies relative to the number of allocated sub-frames. Themethod further comprises transmitting, from the wireless communicationdevice, the data packet according to the defined physical layercharacteristics in the one or more allocated sub-frames.

Another exemplary embodiment provides a wireless communication deviceconfigured to transmit data using one or more allocated sub-frames of atransmission time interval (TTI). The wireless communication device(100) comprising an allocation circuit, a resource defining circuit, anda transmitter. The allocation circuit is configured to allocate one ormore sub-frames to a data packet. The resource defining circuit isconfigured to define one or more physical layer characteristics of theallocated sub-frames based on the number of allocated sub-frames. Theone or more physical layer characteristics comprise at least one of areference signal density, a sub-frame structure, and a data rateparameter. The reference signal density identifies a number of referencesignals allocated to the data packet in response to the number ofallocated sub-frames. The reference signal density variesdisproportionately relative to the number of allocated sub-frames as thenumber of allocated sub-frames varies. The sub-frame structureidentifies a distribution of one or more reference signals within thedata packet in response to the number of allocated sub-frames. The datarate parameter identifies at least one of a coding rate and a ratematching of the data packet in response to the number of allocatedsub-frames. The data rate parameter varies relative to the number ofallocated sub-frames. The transmitter is configured to transmit the datapacket according to the defined physical layer characteristics in theone or more allocated sub-frames.

Another exemplary embodiment provides a wireless communication deviceconfigured to transmit data using one or more allocated sub-frames. Thewireless communication device comprising at least one processing circuitand at least one memory operatively connected to the at least oneprocessing circuit. Using the at least one processing circuit and the atleast one memory, the wireless communication device is operative toallocate one or more sub-frames to a data packet by defining one or morephysical layer characteristics of the allocated sub-frames based on thenumber of allocated sub-frames. The one or more physical layercharacteristics comprise at least one of a reference signal density, asub-frame structure, and a data rate parameter. The reference signaldensity identifies a number of reference signals allocated to the datapacket in response to the number of allocated sub-frames. The referencesignal density varies disproportionately relative to the number ofallocated sub-frames as the number of allocated sub-frames varies. Thesub-frame structure identifies a distribution of one or more referencesignals within the data packet in response to the number of allocatedsub-frames. The data rate parameter identifies at least one of a codingrate and a rate matching of the data packet in response to the number ofallocated sub-frames. The data rate parameter varies relative to thenumber of allocated sub-frames. The wireless communication device isfurther operative to transmit the data packet according to the definedphysical layer characteristics in the one or more allocated sub-frames.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary wireless communication system.

FIGS. 2A-2B show exemplary fixed and flexible TTI approaches.

FIG. 3 shows an exemplary flexible Transmission Time Interval (TTI)frame structure.

FIGS. 4A-4B show a conventional reference signal distribution for asystem with frequency and time multiplexing, e.g., an OrthogonalFrequency Division Multiple Access (OFDMA) system in Long Term Evolution(LTE) networks.

FIG. 5 shows a flexible TTI structure using a conventional referencesignal distribution.

FIGS. 6A-6C show exemplary Atomic Scheduling Units (ASUs).

FIG. 7 shows an exemplary method of reference signal distribution withina transmitted data packet according to an exemplary embodiment.

FIG. 8 shows a wireless communication device according to one exemplaryembodiment.

FIG. 9 shows a flexible TTI structure using a flexible reference signaldistribution according to one exemplary embodiment.

FIG. 10 shows another flexible TTI structure using a flexible referencesignal distribution according to another exemplary embodiment.

FIG. 11 shows another flexible TTI structure using a flexible referencesignal distribution according to another exemplary embodiment.

FIG. 12 shows another flexible TTI structure using a flexible referencesignal distribution according to another exemplary embodiment.

FIG. 13 shows another flexible TTI structure using a flexible referencesignal distribution according to another exemplary embodiment.

FIG. 14 shows another flexible TTI structure using a flexible referencesignal distribution according to another exemplary embodiment.

FIG. 15 shows another flexible TTI structure using a flexible referencesignal distribution according to another exemplary embodiment.

DETAILED DESCRIPTION

The solution presented herein facilitates a more efficient use of timeand frequency resources by variably defining the physical layercharacteristics of the time and/or frequency resources allocated to adata packet. To facilitate the detailed description of the solution, thefollowing first explains the terminology and system components that formthe backdrop for this solution.

In wireless systems, e.g., the exemplary wireless system 10 shown inFIG. 1, a transmitter 12 transmits data with one or more referencesignals to a remote device, e.g., remote receiver 14, via a wirelessradio channel 16. As discussed previously, reference signals comprisethose signals transmitted by the transmitter 12 and known by thereceiver 14, e.g., pilot signals, that enable the remote device toestimate the radio channel 16.

The transmitter 12 discussed herein may be comprised in an access pointwithin a wireless communication network that provides a radiocommunication device with access to the wireless communication network.Exemplary access points include, but are not limited to, an access node,a NodeB, an enhanced NodeB (eNodeB), and a base station. The receiver 14discussed herein comprises a wireless receiver, which may be comprisedin a radio communication device. Exemplary radio communication devicesinclude but are not limited to, a user equipment (UE), a mobileterminal, a terminal, sensors and/or actuators with wirelesscapabilities, and a machine with wireless capabilities. While thesolution presented herein is generally described in terms of downlinktransmissions from an access point to a radio communication device, thesolution presented herein applies equally well to uplink transmissionsfrom the radio communication device to the access point. Thus,transmitter 12 and receiver 14 comprise any wireless transmitter andreceiver pair that exchange wireless signals.

In conventional wireless systems, data packets are transmitted in one ormore Transmit Time Intervals (TTIs) from the transmitter 12 to thereceiver 14, where each TTI has a fixed duration. The minimumaddressable scheduling unit is called an atomic scheduling unit (ASU).In the time domain, one ASU is equivalent to a TTI for conventionalsystems, where the TTI may contain one or more sub-frames. In thefrequency domain, one ASU is equivalent to the smallest subunit of thesystem bandwidth that can be allocated to a user. During resourceallocation to a user, the frequency domain may be partitioned intomultiple frequency domain ASUs, where in some cases the entire systembandwidth (e.g., all of the frequency domain ASUs) in one sub-frame areallocated to one user. As a result, when the size of a data packetcontaining the data and reference signal(s) does not fit into a singleTTI, the data packet must be partitioned into several TTIs. Referencesignals are normally defined in the physical layer based on the numberof ASU(s) according to a predetermined and fixed reference signalallocation plan. Thus, each conventionally allocated ASU of a TTI willhave a fixed number of reference signals at fixed locations within theASU.

One ASU represents the minimum physical layer frequency and/or timeresource unit available for allocation to a data packet. For example, inthe time domain of conventional systems, one ASU is equivalent to oneTTI (the minimum time resource unit in a frame available for allocationto a data packet), while in the frequency domain, one ASU is equivalentto the minimum bandwidth available for allocation to the data packet, asshown in FIG. 6A. Based on the size of the data packet to be transmitted(including the necessary control signaling and overheads introduced byreference signals, etc.), a certain number of ASUs in both the frequencyand time domains are allocated in the TTI used to transmit the datapacket, and thus are allocated to the data packet. For example, FIG. 6Bshows a TTI spanning three ASUs. In a conventional system, all threeASUs will be allocated to a data packet whenever this TTI is allocatedto that data packet. FIG. 6C shows another example, where two ASUs inthe frequency domain and two ASUs in the time domain are allocated to adata packet, where the TTI spans two ASUs. In any event, conventionalsolutions predefine a fixed number of reference signals for each ASU,where each reference signal is distributed throughout each ASU accordingto the same predefined distribution plan (e.g., as shown in FIGS. 4A and4B, which are further discussed below). The solution presented hereinmore flexibly defines the physical layer characteristics, whichtypically results in a different number of reference signals, e.g.,fewer reference signals, than would be present in a conventionalsolution. Thus, the solution presented herein more efficiently uses theavailable time and frequency domain resources of each allocated ASU.

With the data rates envisioned by Ultra-Dense Networks (UDN), e.g., onthe order of 10 Gbps, it becomes feasible to convey complete IP packetswithout partitioning the data packet into several TTIs through the useof “flexible” TTIs. Simpler protocol structures are typically the resultof avoiding such partitioning. IP packets vary in size, and today themost common IP packet sizes are several tens of bytes (IP controlpackets) and 1500 bytes (Maximum Transmission Unit (MTU) size ofEthernet). Flexible TTIs therefore provide the ability to fit an IPpacket into one TTI. For example, such flexible TTIs may be shorter inthe time domain for small IP packets and longer for large IP packets.With such flexible TTI solutions, the size of a TTI is flexible, and canbe fit to the size of the data packet to be transmitted, where thereference signals in the physical layer are allocated accordingly (i.e.,according to the predetermined and fixed resource allocation plandesignated for each ASU). U.S. Provisional Application Ser. No.61/877,444, which is incorporated by reference herein, and which wasfiled concurrently with the Provisional Application of the instantapplication, discloses exemplary flexible TTI solutions. Such flexibleTTI solutions shorten the transmission time of packet data, enablelonger sleep time, and increase power efficiency of wirelesscommunication devices (e.g., mobile terminals, user equipment, laptopcomputers, etc.) and access points (e.g., access nodes, base stations,etc.).

FIGS. 2A-2B show examples of the fixed (FIG. 2A) and flexible (FIG. 2B)TTI approaches in a simplified way in only the time domain, i.e., a TimeDivision Multiplexing (TDM) approach. The flexible TTI solution (FIG.2B) allows a shorter transmission time than that of the fixed TTIsolution (FIG. 2B) because the flexible TTI only includes thosesub-frames necessary to transmit the data packet (FIG. 2A uses a patternof dots to show the data packet length). FIG. 3 shows an exemplaryflexible TTI structure. FIG. 3 only shows flexibility in the time-domainand does not show resource granularity in the frequency-domain. Note,however, that the flexible TTI supports a Frequency Division Multiplex(FDM) component, where within a given subframe, a user may also beassigned a fraction of the complete bandwidth. A frame in the example ofFIG. 3 is 100 μs long and is subdivided into 8 subframes, each 12.5 μslong. In the frequency-domain, the system bandwidth may, e.g., bedivided into 100 MHz sub-channels.

As shown in FIG. 3, each frame starts with Zone 1, which is used for thetransmission of control information (shown in Zone 1 by the mediumdensity dotted pattern). Zones 2 and 3 respectively follow Zone 1, andrepresent the receive and transmit parts of the frame, respectively,from the point of view of the remote receiver 14 in a remote wirelesscommunication device. In FIG. 3, as well as FIGS. 5 and 9-15, darkblocks with a dense dotted pattern are used to represent referencesignals in Zones 1 and 2, while lined patterns are used to represent thedata in the corresponding Zone 2 sub-frame(s).

The control signaling in Zone 1 contains information regarding whichresources within Zone 2 the remote wireless communication device shoulddecode and which resources the remote wireless communication device mayuse for transmission. The control signaling in Zone 1 may furthercontain acknowledgement bits from a node, e.g., a wireless communicationdevice containing the transmitter 12, that received transmissions fromthe remote wireless communication device in an earlier frame.

Zone 2 contains data transmitted to the remote receiver 14 of the remotewireless communication device. Depending on the amount of data in a datapacket, one, a few, or all sub-frames within Zone 2 may be assigned tothe remote wireless communication device. If the assigned resources forone frame are too few to convey a complete data packet, flexible TTI mayeven extend into the next frame, and sub-frames from the next frame maybe used. The time duration of the allocated sub-frames define theFlexible TTI length. The minimum scheduling unit in the example of FIG.3 is 1 subframe (time)×1 sub-channel (frequency)=12.5 μs×100 MHz. Asnoted above, such a minimum scheduling unit (in both time and frequency)is also referred to herein as an Atomic Scheduling Unit (ASU). With arather low spectrum efficiency of 1 bit/s/Hz one ASU can support 1250bit≈150 byte.

If a remote wireless communication device has resources assigned in Zone3, the radio communication device containing the receiver 14 can usethese assigned resources to transmit data, e.g., to the wirelesscommunication device containing the transmitter 12. As with the Zone 2resources, the Zone 3 resources are presented in multiples of onesub-frame. In addition to user data transmissions, acknowledgment bitsin response to received data (either in this frame, or if nodeprocessing is too slow, in response to data received in an earlierframe) may be transmitted in Zone 3. As shown in FIG. 3, the Zone 2/Zone3 split is flexible.

As noted above, reference signals are conventionally defined accordingto a predefined and fixed allocation plan. This is because inconventional wireless communication systems, e.g., in the LTE system,there is little flexibility in terms of the time and frequency resourceallocation for reference signals. Channel estimates for data attime-frequency positions without reference signals are then obtained byinterpolation and extrapolation (in time and/or frequency) from thechannel estimates determined for the time-frequency positions containingthe reference signals. In general, the conventional number of referencesignals embedded into a TTI does not depend on the length of the TTI.Rather, time and frequency resource elements are reserved in advance forreference signals, where in conventional solutions these reservedreference signal resources within an ASU are fixed in both location anddensity. For example, FIG. 4 shows a conventional reference signaldistribution for a system with frequency and time multiplexing, e.g., anOrthogonal Frequency Division Multiple Access (OFDMA) system in LongTerm Evolution (LTE) networks. In FIG. 4, the dark blocks with a densedotted pattern show areas within a sub-frame reserved for referencesignals, where FIG. 4A shows one time-domain ASU in one TTI, and FIG. 4Bshows two time-domain ASUs for two TTIs. FIG. 5 shows anotherconventional reference signal distribution for a system utilizing aflexible TTI solution. In FIG. 5, the smaller blocks having a densedotted pattern represent time and frequency domain resources within thesub-frames reserved for reference signals. The lined patterns and theless dense dotted pattern in Zones 2 and 3 represent the data in thecorresponding sub-frame, where different patterns are given to differenttypes of data, e.g., downlink (DL), uplink (UL), and/or ACK/NACK. InZone 1, the medium density dotted pattern represents the controlsignaling discussed above. As shown by both FIGS. 4 and 5, the number ofreference signals defined by conventional solutions, which always usesthe same time and frequency resources in each ASU, increasesproportionally with the number of allocated ASUs. While thisdistribution provides a sufficient number of reference signals to allowaccurate recovery of the transmitted data, such a proportionalallocation of the reference signals may consume an unnecessary number oftime and frequency resources.

The solution presented herein statically or semi-statically definesresources, including one or more physical layer characteristics of thesub-frame(s) allocated to a data packet, based on the number ofsub-frames allocated to the data packet. The allocated sub-frames arepreferably, but not necessarily, scheduled contiguously.

FIG. 7 shows one exemplary method 50 of transmitting a data packetallocated one or more sub-frames, as executed by a wirelesscommunication device 100 (FIG. 8). The physical layer characteristics ofeach sub-frame are defined based on the number of allocated sub-frames.Accordingly, the wireless communication device 100 allocates one or moresub-frames to a data packet for transmission to a remote device 14(block 52). Based on the number of allocated sub-frames, the wirelesscommunication device 100 defines one or more physical layercharacteristics of the allocated sub-frames (block 54). The wirelesscommunication device 100 then transmits the data packet according to thedefined physical layer characteristics in the allocated sub-frame(s)(block 56). In so doing, the solution presented herein provides asufficient amount of resources needed to meet signal qualityrequirements without overburdening the system.

FIG. 8 shows an exemplary wireless communication device 100. Wirelesscommunication device 100 comprises one or more processing circuits 110,one or more memories 120, a transceiver 130, and at least one antenna140. These components operate together to execute the method 50 of FIG.7.

In one exemplary embodiment, processing circuit 110 comprises anallocation circuit 112 and a resource defining circuit 114. Theprocessing circuit 110 may also optionally include a channel conditioncircuit 116, discussed further below. The allocation circuit 112 isconfigured to allocate one or more sub-frames to a data packet. Thisallocation may be made according to a flexible TTI solution. Theresource defining circuit 114 is configured to define one or morephysical layer characteristics of the allocated sub-frame(s) based onthe number of allocated sub-frame(s), where the physical layercharacteristic(s) comprise at least one of a reference signal density, asub-frame structure, and a data rate parameter. The transceiver 130comprises a transmitter 132 configured to transmit wireless signals to aremote device 14 via antenna 140, and a receiver 134 configured toreceive wireless signals via antenna 140. In particular, the transmitteris configured to transmit the data packet according to the definedphysical layer characteristics in one or more allocated sub-frames.

The resource defining circuit 114 may define the physical layercharacteristics based on one or more tables stored in memory 120 thatcross-reference different physical layer characteristic options withdifferent numbers of sub-frames. Alternatively, resource definingcircuit 114 may define the physical layer characteristics according toan algorithm dependent on the number of sub-frames. In yet anotherembodiment, the resource defining circuit 114 may receive physical layercharacteristic options from a remote network node (not shown) based onthe number of allocated sub-frames provided to the remote network node,and subsequently select one option to define the physical layercharacteristics.

The physical layer characteristics defined by the resource definingcircuit 114 may include a reference signal density, a reference signaldistribution, and/or a data rate parameter. The reference signal densityidentifies a number of reference signals allocated to the data packet. Asub-frame structure identifies the reference signal distribution withinthe data packet. The data rate parameter identifies at least one of acoding rate and a rate matching of the data in the data packet.

As discussed above, the resource defining circuit 114 defines thephysical layer characteristic(s) based on a number of allocatedsub-frames. The physical layer characteristics may be defined byincreasing or decreasing the number of reference signals, changing thereference signal distribution, changing the data rate parameter, etc.,all based on the number of allocated sub-frames. In one exemplaryembodiment, the resource defining circuit 114 defines the physical layercharacteristics by disproportionately increasing the number of referencesignals allocated to the data packet as the number of allocatedsub-frames increases. In another exemplary embodiment, the resourcedefining circuit 114 defines the physical layer characteristics byvarying a distribution of the plurality of reference signals in one ormore of the allocated sub-frames based on the number of allocatedsub-frames as the number of allocated sub-frames varies. In stillanother exemplary embodiment, the resource defining circuit 114 definesthe physical layer characteristics by defining the reference signaldensity and the sub-frame structure (i.e., the reference signaldistribution) based on the number of allocated sub-frames. It will beappreciated that the resource defining circuit 114 may define more thanone physical layer characteristics. For example, the resource definingcircuit 114 may define the reference signal density and the referencesignal distribution based on the number of allocated sub-frames.

In one exemplary embodiment, the wireless communication device 100,e.g., the resource defining circuit 114 in device 100, defines a firstphysical layer characteristic for all data packets allocated a minimumnumber of sub-frames, e.g., one or two sub-frames. For data packetsallocated more than the minimum number of sub-frames, the resourcedefining circuit 114 defines a second physical layer characteristic. Insome cases, defining the first and second physical layer characteristicscomprises defining first and second configurations of the same physicallayer characteristic. For example, the resource defining circuit 114 maydefine a first number of reference signals for all data packetsallocated a minimum number of sub-frames, and may define a second numberof reference signals for data packet(s) allocated a second number ofsub-frames exceeding the minimum number of allocated sub-frames. In thiscase, e.g., the first number of reference signals are optimized infrequency and time based on, e.g., a coherence time of a channel and asize of the data block to be transmitted over the channel, where thesecond number of reference signals may be equal to or greater than thefirst number of reference signals. In other cases, defining the firstand second physical layer characteristics comprises defining differentphysical layer characteristics. For example, the resource definingcircuit 114 may define a number of reference signals for all datapackets allocated a minimum number of sub-frames, and may define areference signal distribution for data packet(s) allocated a secondnumber of sub-frames exceeding the minimum number of allocatedsub-frames. While these examples only discuss first and second physicallayer characteristics for two different quantities of allocatedsub-frames, it will be appreciated that these examples may be expandedto include more than two different quantities of allocated sub-frames.For example, the resource defining circuit 114 may define a third numberof reference signals for data packet(s) allocated more than the secondnumber of sub-frames.

The resource defining circuit 114 may also adjust the defined physicallayer characteristics based on other variables, e.g., a channelcondition. For example, the wireless communication device 100 mayfurther include a channel condition circuit 116 configured to determineat least one channel condition associated with the data packet. Theresource defining circuit 114 may then adjust the defined physical layercharacteristics based on the determined channel condition(s). Exemplarychannel conditions include, but are not limited to a speed of thewireless communication device 100, a speed of the remote receiver 14intended to receive the data packet, a carrier frequency (e.g., 3 GHz,20 GHz), a location of the wireless communication device 100 relative toa structure, a location of the remote receiver 14 intended to receivethe data packet relative to a structure, a frequency selectivity of awireless channel between the wireless communication device 100 and theremote device 14, and a Doppler shift of the radio channel. For example,the resource defining circuit 114 may adjust the defined physical layercharacteristics with respect to maximum Doppler spread due to mobility,e.g., may increase the number of reference signals as the speedincreases. In another example, the resource defining circuit 114 mayadjust the defined physical layer characteristic(s) based on anavailable transmission power of the wireless communication device 100, acapability of the remote receiver 14 (e.g., whether receiver 14 has asophisticated or simple receiver algorithm), and/or a local systemload/current interference. For example, the resource defining circuit114 may adjust the defined physical layer characteristics by adding morereference signals in power limited situations. The adjustment and/ordefinition of the physical layer characteristics provided by theresource defining circuit 114 may be applied statically orsemi-statically with deployment (e.g., indoor, outdoor, urban, highway,carrier frequency, etc.).

For simplicity, most of the following examples illustrate the solutionpresented herein in terms of reference signal density and/or referencesignal distribution. For example, one exemplary method includes defininga reference signal density identifying a number of reference signalsallocated to the data packet based on the number of allocatedsub-frames, where the reference signal density varies disproportionatelyrelative to the number of allocated sub-frames as the number ofallocated sub-frames varies. It will be appreciated, however, that thesolution presented herein applies equally well to other physical layercharacteristics. While much of the solution presented herein isdescribed in terms of reference signals, the reference signals mentionedherein may also be referred to as pilot signals, reference/pilotsymbols, or any other signal or symbol known to the receiver andtransmitted by a transmitter to help the receiver accurately estimatethe radio channel and/or recover the transmitted data.

FIGS. 9-14 show several examples of variations to the reference signaldensity and/or distribution responsive to variations in the number ofallocated sub-frames. Each of these examples shows a flexible TTI(having a variable length) used to transmit a data packet. The flexibleTTI includes the previously discussed zones (e.g., Zones 1-3). As usedherein, the sub-frames allocated to a data packet for transmission bytransmitter 12 and reception by remote receiver 14 refer to thesub-frames in Zone 2 (i.e., the downlink sub-frames). As discussedpreviously, the smaller blocks having a dense dotted pattern representtime and frequency domain resources within the sub-frames in Zones 1 and2 that are reserved for reference signals. The lined patterns in Zone 2represent the data in the corresponding sub-frame, where different linedpatterns are given to different users. In Zone 1, the medium densitydotted pattern represents the control signaling discussed above.

FIG. 9 shows an example where the data packet is allocated only onesub-frame in Zone 2. When only one sub-frame is allocated to a datapacket (as in FIG. 9), the allocated sub-frame contains some minimumnumber of reference signals. However, when two sub-frames are allocatedto a data packet, it may be sufficient to have only reference signals(e.g., the minimum number of reference signals) in one of the allocatedsub-frames, as shown in FIG. 10. In this example, channel estimatesobtained based on the reference signals in the first sub-frame arereused or extrapolated for the second sub-frame. Thus, the secondsub-frame does not need to include any reference signals. The accuracyof the reuse/extrapolation of the channel estimates from the firstsub-frame for the second sub-frame may be improved if the referencesignals in the first sub-frame are distributed towards a middle of theallocated sub-frames, e.g., towards the end of the first sub-frame.Thus, the example of FIG. 10 varies the reference signal density anddistribution based on the number of allocated sub-frames. Because onlyone of the two allocated sub-frames contains reference signals, theexample of FIG. 10 reduces the reference signal overhead, and thusreduces the system overhead.

FIG. 11 shows another example, where only two of the three sub-framesallocated to the data packet contain reference signals. In this example,channel estimates obtained based on the reference signals in the firstand/or third sub-frames are reused or interpolated for the secondsub-frame. Thus, the second sub-frame does not need to include anyreference signals. The accuracy of the reuse/interpolation of thechannel estimates from the first and/or third sub-frames for the secondsub-frame may be improved if the reference signals in the first and/orthird sub-frames are distributed along the edges of their respectivesub-frames so as to be closest to the middle sub-frame. Like the exampleof FIG. 10, the example of FIG. 11 reduces reference signal overhead,and thus reduces system overhead relative to conventional solutions.

FIG. 12 shows yet another example, where only one of the threetime-domain sub-frames allocated to the data packet contains referencesignals. In this example, channel estimates obtained based on thereference signals in the second sub-frame are reused or extrapolated forthe first and third sub-frames. Thus, the first and third sub-frames donot need to include any reference signals. The accuracy of thereuse/extrapolation of the channel estimates from the second sub-framefor the first and third sub-frames may be improved if the referencesignals in the second sub-frame are distributed towards a middle of themiddle sub-frames, and thus along a middle of the data packet. Thisexample shows how the reference signal overhead can be even furtherreduced relative to the example of FIG. 11. The solution of FIG. 12 maybe best, e.g., when the relative speed between the transmitter 12 andreceiver 14 is low, causing the channel to vary slowly.

FIG. 13 shows another example where both of the allocated sub-framescontain reference signals, but each allocated sub-frame has a differentdistribution of the reference signals. This example does not reduce thenumber of reference signals defined for the data packet, and thus doesnot reduce the reference signal overhead. However, by distributing thereference signals as shown, e.g., at opposing edges of the allocatedsub-frames, this example avoids extrapolation towards the outer edges ofthe allocated sub-frames, and thus provides a more accurate solution.The solution of FIG. 13 may be better, e.g., when channel conditions arebad, in which cases extrapolation does not work.

FIG. 14 shows an example where one TTI includes data packets for twodifferent users. In this case, the data packet for User 1 is allocatedthree sub-frames and the data packet for User 2 is allocated onesub-frame. According to the solution presented herein, differentphysical layer characteristics may be defined for the User 1 datapackets than are defined for the User 2 data packets. In this example,the resource defining circuit 114 defines the reference signal densityand distribution for User 1 the same as shown in FIG. 11, and definesthe reference signal density and distribution for User 2 the same asshown in FIG. 9. It will be appreciated that the multi-user solution isnot limited to the specific example of FIG. 14.

The examples of FIGS. 9-14 only use TDM to multiplex different users.Thus, each of these examples allocates the entire available frequencydomain ASUs, and thus the entire system bandwidth, to each data packet.FIG. 15 shows an exemplary embodiment that uses TDM and FDM whenallocating ASUs to different users, where the data for different usersis shown using different lined patterns. This allocation frees upadditional time and frequency domain resources for additional datapackets in Zone 2, e.g., for User 3 and User 4. As shown in FIG. 15, thedefined physical layer characteristics (e.g., reference signal density,reference signal distribution, and/or data parameters) for each datapacket may be different. In this case, the channel can be frequencyselective, e.g., User 1 and/or User 2 may have a good channel at somefrequencies (e.g., the lower system bandwidth frequencies) but a worsechannel at the other frequencies (e.g., the higher system bandwidthfrequencies). The opposite may be true for User 3 and/or User 4. A pureTDM solution would spread out the data across the entire systembandwidth. Using only the “good” frequencies for each user, as shown inFIG. 15, reduces the amount of required resources.

While each of the examples of FIGS. 9-15 show the same reference signalposition in the frequency domain for each ASU that includes referencesignals, it will be appreciated that this is done only for simpleillustration. The reference signal distribution may vary in time and/orfrequency for each ASU containing reference signals.

The solution presented herein reduces, in some cases, the number ofresources needed for reference signals as a function of the flexible TTIlength. Thus, more resources are available for data transmission. Forexample, FIG. 10 shows a solution that allows more data to be includedin the second sub-frame than would have been possible with aconventional reference signal density and distribution, e.g., that ofFIG. 5. As another example, FIG. 11 shows a solution that frees up moretime and frequency resources in the second sub-frame than would havebeen available with a conventional reference signaldensity/distribution, while FIG. 12 shows a solution that frees up evenmore time/frequency resources, e.g., in the first and third sub-frames.As a result, other physical layer characteristics particular to the datain the data packet, i.e., data rate parameters, may also be definedbased on the number of allocated sub-frames. Such data rate parametersinclude, but are not limited to, the code rate and/or rate matching ofthe data. For example, the coded bits in a sequence of coded bits (e.g.,[x1 x2 x3 x4 x5 . . . ] included with the transmission may varydependent on the number of allocated sub-frames, e.g., only thefollowing sequence may be transmitted: [x1 x2 x4 x5 x6 x8 . . . ].

As discussed above, the solution presented herein may be implemented bya wireless communication device 100 comprising one or more processingcircuits 110 and at least one memory 120. Alternatively, the solutionmay be implemented by a computer program product, e.g., stored in anon-transitory computer readable medium. In this case, the computerprogram product comprises software instructions that control thewireless communication device 100 such that when the softwareinstructions are run on the wireless communication device 100, thewireless communication device 100 defines one or more physical layercharacteristics of the allocated sub-frames based on the number ofallocated sub-frames, and transmits the data packet according to thedefined physical layer characteristics in the one or more allocatedsub-frames. A carrier containing such a computer program product maycomprise an electronic signal, an optical signal, a radio signal, or acomputer readable storage medium.

Various elements disclosed herein are described as some kind of circuit,e.g., a processing circuit, an allocation circuit, a channel conditioncircuit, a resource defining circuit, etc. Each of these circuits may beembodied in hardware and/or in software (including firmware, residentsoftware, microcode, etc.) executed on a controller or processor,including an application specific integrated circuit (ASIC).

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive, and allchanges coming within the meaning and equivalency range of the appendedclaims are intended to be embraced therein.

What is claimed is:
 1. A method of transmitting data from a wirelesscommunication device to a remote device using one or more allocatedsub-frames of a transmission time interval (TTI), the method comprising:allocating one or more sub-frames to a data packet; defining a physicallayer characteristic of the allocated one or more sub-frames based onthe number of allocated sub-frames, wherein the physical layercharacteristic comprises: a reference signal density identifying anumber of reference signals allocated to the data packet in response tothe number of allocated sub-frames, wherein the number of referencesignals allocated to the data packet increases disproportionately as thenumber of allocated sub-frames increases; and transmitting, from thewireless communication device, the data packet according to the definedphysical layer characteristic in the one or more allocated sub-frames.2. The method of claim 1, wherein defining the physical layercharacteristic comprises: defining a first configuration of the physicallayer characteristic for data packets allocated at least a minimumnumber of sub-frames; and defining a second configuration of thephysical layer characteristic for data packets allocated more than theminimum number of sub-frames.
 3. The method of claim 1, wherein definingthe physical layer characteristic comprises varying the distribution ofthe one or more reference signals in one or more of the allocatedsub-frames based on the number of allocated sub-frames as the number ofallocated sub-frames varies.
 4. The method of claim 1, furthercomprising: determining at least one channel condition associated withthe data packet; and adjusting the defined physical layer characteristicbased on the at least one determined channel condition.
 5. The method ofclaim 4, wherein determining the at least one channel conditioncomprises determining at least one of a speed of the wirelesscommunication device and a speed of a remote receiver in the remotedevice intended to receive the data packet.
 6. The method of claim 4,wherein determining the at least one channel condition comprisesdetermining at least one of: a carrier frequency; a frequencyselectivity of a wireless channel between the wireless communicationdevice and the remote device; and a Doppler shift of the wirelesschannel.
 7. The method of claim 1, further comprising adjusting thedefined physical layer characteristic based on an available transmissionpower of the wireless communication device.
 8. The method of claim 1,further comprising adjusting the defined physical layer characteristicbased on a capability of a remote receiver in the remote device intendedto receive the data packet.
 9. The method of claim 1, further comprisingadjusting the defined physical layer characteristic based on a localsystem load.
 10. A wireless communication device configured to transmitdata using one or more allocated sub-frames of a transmission timeinterval (TTI), the wireless communication device comprising: anallocation circuit configured to allocate one or more sub-frames to adata packet; a resource defining circuit configured to define a physicallayer characteristic of the allocated one or more sub-frames based onthe number of allocated sub-frames, wherein the physical layercharacteristic comprises: a reference signal density identifying anumber of reference signals allocated to the data packet in response tothe number of allocated sub-frames, wherein the number of referencesignals allocated to the data packet increases disproportionately as thenumber of allocated sub-frames increases; and a transmitter configuredto transmit the data packet according to the defined physical layercharacteristic in the one or more allocated sub-frames.
 11. A wirelesscommunication device configured to transmit data to a remote deviceusing one or more allocated sub-frames of a transmission time interval(TTI), the wireless communication device comprising: memory; one or moreprocessing circuits operatively connected to the memory and configuredto allocate one or more sub-frames to a data packet by: defining aphysical layer characteristic of the allocated one or more sub-framesbased on the number of allocated sub-frames, wherein the physical layercharacteristic comprises: a reference signal density identifying anumber of reference signals allocated to the data packet in response tothe number of allocated sub-frames, wherein the number of referencesignals allocated to the data packet increases disproportionately as thenumber of allocated sub-frames increases; and a transmitter configuredto transmit the data packet according to the defined physical layercharacteristic in the one or more allocated sub-frames.
 12. The wirelesscommunication device of claim 11, wherein the one or more processingcircuits are configured to define the physical layer characteristic by:defining the physical layer characteristic for data packets allocated atleast a minimum number of sub-frames; and defining a second additionalphysical layer characteristic for data packets allocated more than theminimum number of sub-frames.
 13. The wireless communication device ofclaim 11, wherein the one or more processing circuits are configured todefine the physical layer characteristic by varying the distribution ofthe one or more reference signals in one or more of the allocatedsub-frames based on the number of allocated sub-frames as the number ofallocated sub-frames varies.
 14. The wireless communication device ofclaim 11, wherein the one or more processing circuits are configured to:determine at least one channel condition associated with the datapacket; and adjust the defined physical layer characteristic based onthe at least one determined channel condition.
 15. The wirelesscommunication device of claim 14, wherein the one or more processingcircuits are configured to determine the at least one channel conditionby determining at least one of a speed of the wireless communicationdevice and a speed of a remote receiver in the remote device intended toreceive the data packet.
 16. The wireless communication device of claim14, wherein the one or more processing circuits are configured todetermine the at least one channel condition by determining at least oneof: a carrier frequency; a frequency selectivity of a wireless channelbetween the wireless communication device and the remote device; and aDoppler shift of the wireless channel.
 17. The wireless communicationdevice of claim 11, wherein the one or more processing circuits areconfigured to adjust the defined physical layer characteristic based onan available transmission power of the wireless communication device.18. The wireless communication device of claim 11, wherein the one ormore processing circuits are configured to adjust the defined physicallayer characteristic based on a capability of a remote receiver in theremote device intended to receive the data packet.
 19. The wirelesscommunication device of claim 11, wherein the one or more processingcircuits are configured to adjust the defined physical layercharacteristic based on a local system load.
 20. A computer programproduct stored in a non-transitory computer readable medium forcontrolling transmission of data from a wireless communication device toa remote device using one or more allocated sub-frames of a transmissiontime interval (TTI), the computer program product comprising softwareinstructions which, when run on one or more processing circuits of thewireless communication device, causes the wireless communication deviceto: allocate one or more sub-frames to a data packet; define a physicallayer characteristic of the allocated one or more sub-frames based onthe number of allocated sub-frames, wherein the physical layercharacteristic comprises a reference signal density identifying a numberof reference signals allocated to the data packet in response to thenumber of allocated sub-frames, wherein the number of reference signalsallocated to the data packet increases disproportionately as the numberof allocated sub-frames increases; and transmit, from the wirelesscommunication device, the data packet according to the defined physicallayer characteristic in the one or more allocated sub-frames.